I have also seen that the orientation of some models is not correct. I have no clue on how the correct orientation can be determined.

Maybe there is a problem in assimp library. The open3mod application that you mentioned uses a very old version of assimp library and there the model is correctly oriented.

But a newer assimp viewer that comes with assimp library also have some problems with orientation - see the following screenshot where the person from the model should be visible from the side or from the front, but it is visible from upside down: